1. Field of the Invention
The invention relates to a method and apparatus for extrusion pressing a granulated, preferably powder-metallurgic material, where the material is fed into a cavity, cold-compressed thereby means of a plunger, then sintered by heating and forced through a die.
2. Description of the Prior Art
It is known that one may compress powder-metallurgic materials so that the pellets are first compressed in suitable forms which are then subjected to a following cold or hot extrusion (cf, e.g. Laue/Staenger "Strangpressen. Verfahren-Maschinin-Werkzeuge" [Extruding Presses. Methods-Machines-Tools]. Duseeldorf, 1976 p. 179/180, Section 3.6). It is also known that one may use, instead of such pellets, dose-like containers full of powder which are simultaneously solidified in and pressed out of the containers (cf. e.g. DT-PS 1 758 540). Commonly in this method, before the extrusion process, an intermediate mass (a compressed pellet with or without a hull) must first be formed and only then can an extrusion pressing be carried out in a process physically separate from the intermediate mass forming process. Moreover, when a pellet with a hull is used, it is also necessary to remove the hull after pressing out the pellet.
The forming of rods from powder mixed with plasticizing agents by means of a die is likewise known. The use of the plasticizing agent leads, of course, to a reduction in powder density, where this added agent can wholly or partially vanish during the sintering process, with formation of undesired spaces between the powder grains and resulting effects on the mechanical properties of the product.
Also known is the extrusion pressing of heated powder whereby an end product with good material characteristics can indeed be obtained, although the high temperature utilized places great stress on the tools and creates difficulties in maintaining close tolerances and smooth surfaces. Such methods are therefore not very economical.
No known method has succeeded so far in producing from powder or granules a direct extrusion to a continuous rod with satisfactory properties and economy. Moreover, current practice in the production of prepressed pellets or even of ringlike parts is possible only on hydraulic presses in tools which limit the production length. Such lengths at present are about 103 mm at most. However, even at this relatively short length the desired internal uniformity of the product is prevented by the taper effect and friction losses in the pressing process. This also leads to the result that, in very highly stressed parts, only product lengths of about 50 to 53 mm are used, and in no case, however, is the ratio of 3 times the wall thickness exceeded.